1. Field of the Invention
The present invention relates to a process for preparing siloxanes; more particularly and in a preferred embodiment, the present invention is a direct process for the preparation of cyclic and oligomeric dimethylsiloxanes by the fluidized bed reaction of copper-activated silicon with dimethyl ether and a methyl halide.
2. Description of the Prior Art
Alkylsiloxanes, especially the dimethylsiloxanes, find extensive use in emulsions, antifoams and pastes on account of their low surface tension, high surface activity, good spreading power, chemical inertness and thermal stability.
The direct synthesis of the dimethylsiloxanes from dimethyl ether and suitably activated silicon has been of interest for many years. This has been so because a successful synthesis would reduce the cost and complexity associated with the state-of-the-art process. Currently, a mixture of methylchlorosilanes is made by the Rochow Direct Reaction (see, e.g., U.S. Pat. No. 2,380,995). This mixture is separated via a multiplate distillation to obtain high purity dimethyldichlorosilane, which is subsequently hydrolyzed to cyclic and linear dimethylsiloxanes and HCl (equation 1): EQU n(CH.sub.3).sub.2 SiCl.sub.2 +nH.sub.2 O.fwdarw.[(CH.sub.3).sub.2 SiO].sub.n +2n HCl (1)
n=3, 4, 5, 6, 7, 8.
Descriptions of the state-of-the-art technology are given in the monographs
W. Noll, "Chemistry and Technology of Silicones", Academic Press, N.Y. (1968); PA0 A. Petrov et. al., "Synthesis of Organosilicon Monomers", Consultants Bureau, N.Y. (1976); and PA0 R. J. H. Voorhoeve, "Methylchlorosilanes: Precursors to Silicones", Elsevier, N.Y. (1967); PA0 W. Buechner, "Organometallic Chemistry Reviews", Library 9, pp 409-431.
and in the review
Corrosion and pollution problems attend the handling and recovery of the liberated HCl. However, the gas may be recycled to the synthesis of HSiCl.sub.3 or used to prepare CH.sub.3 Cl from CH.sub.3 OH for the Rochow Direct Reaction. Some silicones manufacturers use methanol in place of water and obtain CH.sub.3 Cl as the by-product (equation 2): EQU n(CH.sub.3).sub.2 SiCl.sub.2 +2nCH.sub.3 OH.fwdarw.[(CH.sub.3).sub.2 SiO].sub.n +2nCH.sub.3 Cl+nH.sub.2 O (2)
Small amounts of HCl and (CH.sub.3).sub.2 O are also formed, and if not removed, have undesirable effects on the rate and selectivity of the Rochow Direct Reaction (i.e., on the synthesis of the methylchlorosilanes). It is to obviate these complexities that there has been much interest in obtaining the direct synthesis of dimethylsiloxanes from silicon and dimethyl ether (equation 3): EQU n(CH.sub.3).sub.2 O+nSi.fwdarw.[(CH.sub.3).sub.2 SiO)].sub.n( 3)
Although not wishing to be bound by theory, the overall process proposed by equation (3) may actually involve the intermediate synthesis of dimethyldimethoxysilane followed by hydrolysis to the cyclic and oligomeric dimethylsiloxanes (equation 4): ##STR1## Methanol can be dehydrated to dimethyl ether (equation 5), which could then be recycled. EQU 2CH.sub.3 OH.fwdarw.(CH.sub.3).sub.2 O+H.sub.2 O (5)
Japanese patent No. 286 (1951) [Chem. Abstr. 47, 3334 (1953)] discloses that dimethyldimethoxysilane and methyltrimethoxysilane were obtained in good yield from the reaction of silicon, containing 10% copper catalyst, and dimethyl ether at 400.degree.-430.degree. C. in a fixed-bed reactor at atmospheric pressure. However, an exhaustive investigation of the reactivity of activated silicon with ethers in fixed-bed reactors by Rochow and Zuckerman ["The Reaction of Ethers with Silicon", PB 157357, NTIS. U.S. Dept. of Commerce] later showed that disclosure to be erroneous (see, e.g., U.S. Pat. No. 4,088,669 at column 1, lines 52-61). Rochow and Newton [Inorg. Chem. 9, 1072 (1970)) also failed to realize methylmethoxysilane formation when the reaction was attempted in a slurry of silicone oil at 300.degree. C. Fredin, et. al., [Abstracts 184th National ACS Meeting, Kansas City, Mo., Mar. 12-17, 1982. #INOR 60] showed that whereas groundstate silicon atoms reacted spontaneously with methanol at 10.degree. K. in an argon matrix, no reaction was observed with dimethyl ether. Photoexcitation of the Si atoms was necessary in order to achieve insertion into the C--O bond of the dimethyl ether.
The co-reaction of mixtures of dimethyl ether and hydrogen chloride with silicon-copper mixtures and contact masses has been disclosed by Rochow (U.S. Pat. No. 2,459,539), by Burgess [British Patent No. 642,997) and by Turetskaya, et. al. (Russ. J. Gen. Chem. 44(12), 2738 (1975)). In each case the principal products were methylchlorosilanes. Presumably, methylchloride was first formed (equation 6) and it then reacted with silicon to give the methylchlorosilanes. EQU (CH.sub.3).sub.2 O+2HCl.fwdarw.2CH.sub.3 Cl+H.sub.2 O (6)
U.S. Pat. No. 4,088,669 discloses that dimethyldimethoxysilane is the main product from the reaction of dimethyl ether and methylbromide with activated silicon at 200.degree.-300.degree. C. under autogenous conditions. The patent specifies that a closed reaction vessel is necessary in order to get dimethyl ether to react with silicon and that activating traces of tin, mercury and copper and long reaction times (1-100 hr) were essential to the success of the synthesis. More particularly, Example 13 of the patent indicates that no reaction product was obtained, under a variety of reaction conditions, when gaseous dimethyl ether was continuously passed through a fixed bed of powdered, copper-activated silicon. The overall process was thought to proceed as described in equations 7-9 [Speier, et. al., Abstracts 13th Organosilicon Symposium, Univ. of Michigan, Mar. 30-31, 1979; see also, column 4, lines 16-22, of U.S. Pat. No. 4,088,669]. The patent describes CH.sub.3 Br as having a catalytic function. ##STR2## The method disclosed in said U.S. Pat. No. 4,088,669 is attended by serious disadvantages when applied on a large scale. The confinement of a large mass of reacting silicon particles in a closed vessel and maintenance of said mass at a uniform temperature between the limits (200.degree.-300.degree. C.) specified therein necessitate special means to obtain adequate heat-transfer and mass transfer through the reaction mixture in order to avoid hot-spots. Hot-spots lead to pyrolysis of the methyl bromide and dimethyl ether and the formation of surface deposits which inhibit further reaction of the silicon mass. Both dimethyl ether (critical temperature 129.degree. C.) and methyl bromide (critical temperature 194.degree. C.) have critical temperatures less than the lower temperature limit set forth in the patent. Consequently, they are both gaseous under the reaction conditions specified. Agitation (e.g., by stirring, rocking or shaking) of the mass is required in order to obtain adequate heat transfer and continually expose fresh surface for reaction. Problems associated with stirring large masses of abrasive silicon particles have been discussed in U.S. Pat. Nos. 2,887,502 and 2,389,931 and are well known to those skilled in the art of the Rochow synthesis. While the use of an inert solvent, as specified in U.S. Pat. No. 4,088,669, reduces the heat and mass transfer problems, said solvent must be free of impurities which inhihit the synthesis of dimethyldimethoxysilane. Recycle of the solvent is essential for the economic practice of this synthesis. Hence additional processing steps (e.g., filtration and distillation) which add to the complexity of the synthesis are required.
Contrary to these teachings, it has now surprisingly been found that the direct synthesis of organosiloxanes and their precursors may be accomplished successfully in a fluidized or agitated bed reactor with contact times substantially less than one hour.